1. Field of The Invention
The present invention relates generally to a heat treatment system and method for heat-treating an object to be treated.
2. Description of Related Art
First, the art related to a first invention provided by the present application will be described below.
As methods for carrying a large number of semiconductor wafers (which will be hereinafter referred to as wafers) in a batch type furnace to oxidize a silicon film on each of the wafers to form a silicon oxide film (SiO2 film), there are known a dry oxidation method using oxygen (O2) gas and hydrogen chloride (HCl) gas, and a wet oxidation method for feeding water vapor and oxygen gas into a reaction tube. The type of the oxidation method is selected in accordance with the quality of an intended film.
In the dry oxidation method, a silicon film is oxidized with oxygen gas, and impurities on the surface are removed by the gettering effect of chlorine. Specifically, for example, after a large number of wafers are held on the shelves of a boat to be carried in a vertical heat treatment furnace to form a treatment atmosphere of a predetermined temperature, oxygen gas and chlorine gas are supplied into a reaction tube from the ceiling portion of the heat treatment furnace at ordinary temperatures, and exhausted from the bottom side. In the wet oxidation method, an external combustion system must be provided outside of a heat treatment furnace. In the external combustion system, part of oxygen gas, and hydrogen (H2O) gas are burned to produce water vapor, and the rest of oxygen gas and water vapor are supplied to a reaction tube. In the above described heat treatment furnace, dinitrogen oxide gas (N2O gas) is fed into a reaction vessel at ordinary temperatures to be allowed to react with a silicon layer on the wafer to produce a nitrogen containing silicon oxide film.
By the way, a defect called slip is more easily caused in the wafer as a process temperature rises. Therefore, in order to avoid the influence of heat on films stacked on an underlayer and in order to save energy, it has been studied to lower the process temperature. However, if the process temperature is lowered, the uniformity of the thickness on the surface of the wafer deteriorates combined with the increase of the diameter of the wafer, and the variation in thickness between wafers (between planes) increases.
After the relationship between the thickness of a silicon oxide film obtained by the dry oxidation method and the position of a wafer on the boat was examined, it was found that the uniformity of the thickness of the film tended to deteriorate as the wafer was positioned on the upper stage side of the boat. The inventor guesses that the reason for this is as follows. FIGS. 6(a), 6(b) and 6(c) schematically show the flow of gases over a wafer W and the temperature and thickness of the wafer W. Oxygen gas and chlorine gas flow from the peripheral edge of the wafer W to the center thereof, and silicon on the wafer is oxidized with oxygen gas. Since heat of the wafer W is radiated from the peripheral edge of the wafer W, the temperature of the wafer W is higher at the center. Thus, the oxidation reaction is accelerated at the center, so that the thickness of the film at the center tends to be originally larger than that at the peripheral edge even if the uniformity of the thickness is high.
On the other hand, hydrogen produced by the decomposition of hydrogen chloride reacts with oxygen to produce a very small amount of water vapor. Since the gases are not sufficiently warmed on the upper stage side of the boat, the amount of produced water vapor increases as the gases are heated from the peripheral edge of the wafer W toward the center thereof. This water vapor serves to increase the thickness of the oxide film, so that the difference in amount of produced water vapor has a great influence on the thickness of the film. As a result, the distribution in thickness is a so-called crest distribution wherein the thickness of the film at the center of the wafer W is large, so that the uniformity of the thickness deteriorates. Then, since the gases are warmed as the gases travel toward the bottom of the reaction tube, the water vapor producing reaction is substantially in an equilibrium state on the lower stage side of the boat, so that water vapor is completely produced before the gases flow along the wafer W. Therefore, when the process gases flow from the peripheral edge of the wafer W toward the center thereof, the amount of water vapor hardly varies regardless of the position of the wafer W, so that the uniformity of the thickness of the film is enhanced. It is thus considered that the uniformity of the thickness of the film is low on the upper stage side of the boat so as to increase the difference in thickness of the film between wafers on the upper and lower stage sides.
Also in the process for producing a nitrogen containing silicon oxide film using dinitrogen oxide gas, the same tendency appears if the process temperature is lowered. That is, dinitrogen oxide gas is decomposed to allow oxygen to react with silicon to produce the nitrogen containing silicon oxide film, and the active species of nitrogen produced by the decomposition of the dinitrogen oxide gas enters the silicon oxide film to grow the nitrogen containing silicon oxide film. The temperature of the wafer W is higher at the center of the wafer W, and the dinitrogen oxide gas is not sufficiently decomposed on the upper stage side of the boat if the process temperature is low. Therefore, as the dinitrogen oxide gas flows toward the center of the wafer W, the decomposition reaction is further accelerated at the center of the wafer w than the peripheral edge thereof, so that the thickness of the film at the center of the wafer W tends to be larger than that at the peripheral edge thereof. Since the dinitrogen oxide gas is warmed as it travels toward the bottom of the reaction tube, the decomposition thereof sufficiently proceeds on the lower stage side of the boat, or the decomposition thereof further proceeds on the lower stage side of the boat than the upper stage side thereof even if it is not sufficient. Thus, the difference in degree of decomposition between the center and peripheral edge of the wafer is small. As a result, the inplane uniformity of the thickness of the film is higher than that on the upper stage side.
Thus, in the present circumstances, the inplane uniformity of the thickness of the film on the wafer is low on the upper stage side, and the uniformity between wafers is low, so that it is difficult to lower the process temperature.
The first invention provided by the present application has been made in such circumstances, and it is an object of the first invention to provide a technique capable of obtaining the high uniformity of the thickness of an oxide film, and contributing to the lowering of a process temperature, when an oxidation process is carried out with respect to an object to be treated.
The art related to a second invention provided by the present application will be described below.
There is a process called the CVD (Chemical Vapor Deposition) as one of deposition processes which are processes for fabricating semiconductor devices. This technique is designed to feed a process gas into a reaction tube to deposit a thin film on the surface of a semiconductor wafer (which will be hereinafter referred to as a wafer) by a chemical gas phase reaction. As one of systems for carrying out such a deposition process in a batch, there is a vertical heat treatment system. As shown in, e.g., FIG. 11, this system comprises a vertical reaction tube 112 provided on a cylindrical manifold 111, a heater 113 provided so as to surround the reaction tube 112, a gas feed pipe 114 extending into the manifold 111, and an exhaust pipe 115 connected to the manifold.
In such a system, a large number of wafers W are held on the shelves of a holder 116 called a wafer boat to be carried in the reaction tube 112 from an opening which is formed in the bottom end of the manifold 111, and a process gas is fed into the reaction tube 112 from a gas supply source 117 via the gas feed pipe 114 to deposit a thin film. At this time, the process gas is heated by the heater 113 in the reaction tube 112 to be decomposed, and further heated to a reaction temperature or higher to carry out a predetermined reaction. The reactant is deposited on the wafer W to form a predetermined film thereon.
By the way, if the film is deposited on the wafer W by means of the above described system, the thickness of the film in the central portion of the wafer tends to be larger than that in the peripheral edge portion thereof as shown in FIG. 12. It is considered that the reason for this is as follows. That is, in the above described vertical heat treatment system called a so-called batch type furnace, a process gas is fed into the reaction tube 112 from the gas feed pipe 114 to be supplied from the peripheral edge portion of the wafer W to the wafer W, which is held on the wafer boat 116, to flow along the wafer from the peripheral portion of the wafer to the central portion thereof, so that the concentration of the process gas in the central portion of the wafer is higher than that in the peripheral portion thereof.
In a process for raising the temperature of the wafer to a process temperature, the heat radiation amount in the peripheral edge portion of the wafer W is greater than that in the central portion thereof, so that the temperature of the central portion of the wafer is higher than that of the peripheral portion thereof. Thus, it is guessed that the deposition reaction is further accelerated in the central portion of the wafer W, in which the temperature and concentration of the process gas are higher, than the peripheral portion thereof due to the differences in temperature and concentration of the process gas between the peripheral and central portions of the wafer W, so that the thickness of the film on the central portion of the wafer W is larger than that on the peripheral portion thereof.
On the other hand, in a semiconductor fabricating process, in order to prevent a bad influence on a film produced at the last step and in order to save energy, a low temperature process is desired. However, the above described phenomenon that the thickness of the film increases in the central portion of the wafer tends to be conspicuous as the process temperature is lowered, so that it is difficult to realize a low temperature process in the existing system.
Therefore, the inventor has studied a technique for lowering a process temperature in the reaction tube 112 by preheating a process gas to a predetermined temperature by means of a heater (not shown), which is provided outside of the reaction tube 112, before feeding the process gas into the reaction tube 112, and feeding the activated and sufficiently heated process gas into the reaction tube 112. For example, the heater comprises a heating chamber for heating a fed process gas, and a heater, provided outside of the heating chamber, for heating the heating chamber. In this technique, since the process gas is preheated by the heater to a temperature approximating to, e.g., a decomposition temperature, the sufficiently activated process gas is fed into a deposition region, and a reaction sufficiently occurs when the process gas reaches the peripheral edge portion of the wafer. Thus, the reaction state in the central portion of the wafer is the same as the reaction state in the peripheral edge portion thereof, so that it is possible to provide the high uniformity of the thickness of the film even if the process temperature in the reaction tube 112 is low.
However, in a low pressure CVD process for reducing the pressure in the reaction tube 112 to carry out a process, the pressure in the heater is also reduced. If the pressure in the heater is reduced to, e.g., about 200 Torr, convection is difficult to occur. In addition, if the pressure in the heater is low, the partial pressure of the process gas decreases, so that heat conduction due to the convection of the process gas in the heater is difficult to occur. Thus, heat is not transferred into the interior of the heater, and the efficiency of heat transfer to the process gas is bad, so that it is difficult to heat the process gas to a temperature at which the process gas is sufficiently activated.
The second invention provided by the present application has been made in such circumstances, and it is an object of the second invention to provide a heat treatment system and method capable of obtaining the high uniformity of the thickness of a formed film and contributing to the lowering of a process temperature by supplying a process gas preheated by a heating part to a reaction vessel, for example, when a thin film is deposited on an object to be treated.
According to one aspect of the present invention, there is provided a heat treatment system comprising a reaction vessel, in which an object to be treated is carried and in which a heat treatment atmosphere at a predetermined temperature is formed, and a combustion system which is provided outside of the reaction vessel, wherein the combustion system allows hydrogen gas and oxygen gas, which pass through a first gas passage and a second gas passage, respectively, to be heated by heating means and to be fed into a combustion chamber to be burned therein to produce water vapor, and wherein gas is fed from the combustion chamber into the reaction vessel to carry out an oxidation process with respect to a silicon layer of the object to form an oxide film thereon, the heat treatment comprising: means for supplying one or more kinds of process gases, which are used for carrying out a process other than the oxidation process using water vapor, to one of the first and second gas passages; and a ventilation resistance material, provided in a region heated by the heating means in one of the gas passages through which the one or more kinds of gases pass, for enhancing the heating efficiency of the gases, wherein the heating means is utilized for heating the process gases to a temperature, at which the process gases are allowed to react or activated, when the process other than the oxidation process using water vapor is carried out with respect to the object by the process gases.
This heat treatment system may further comprise a control part for controlling a heating temperature of the heating means in accordance with a heat treatment which is carried out in the reaction vessel. For example, the reaction vessel corresponds to a reaction tube of a vertical heat treatment furnace. As an example of the process other than the oxidation process using water vapor, there is a process for passing a process gas containing a gas of a compound including hydrogen and chlorine, e.g., hydrogen chloride gas or dichloroethylene gas, and oxygen gas through the second gas passage to heat the process gas by the heating means to produce a very small amount of water vapor to supply the process gas containing the very small amount of water vapor into a heat treatment furnace to carry out an oxidation process with respect to the object. In this case, a temperature to which the process gas containing the gas of the compound including hydrogen and chlorine and oxygen gas is heated by the heating means is preferably higher than a temperature at which the process gas is used for processing the object in the reaction vessel. In this case, since the very small amount of water vapor has been produced when the process gas enters the reaction vessel, the amounts of water vapor on the center and periphery of the object are not so different. Therefore, the difference in degree of the thickness increasing effect due to water vapor decreases, so that the inplane uniformity is enhanced. In addition, the combustion system is utilized for heating the process gas, and this is advantageous to costs and space efficiency.
As another example of the process other than the oxidation process using water vapor, there is a process for passing dinitrogen oxide gas through the first or second gas passage to heat and activate dinitrogen oxide gas by the heating means to supply the activated dinitrogen oxide gas into the reaction vessel to form a nitrogen containing silicon oxide film on the object. In this case, a temperature to which dinitrogen oxide gas is heated by the heating means is preferably higher than a temperature at which dinitrogen oxide gas is used in a heat treatment furnace for processing the object. In this case, since dinitrogen oxide gas has been previously activated, there is no or a little difference in degree of activation due to the difference in place when the gas flows along the surface of the object, so that the inplane uniformity of the thickness is enhanced.
According to another aspect of the present invention, there is provided a heat treatment system wherein an object to be treated is carried in a reaction vessel, which has been pressure-reduced to a predetermined degree of vacuum by evacuating means and the interior of which is heated to a predetermined process temperature, and a process gas is supplied into the reaction vessel via a gas feed passage to process the object, the heat treatment system comprising: a heating part, provided in the gas feed passage, for heating the process gas to a predetermined temperature before the process gas is supplied to the reaction vessel; and an orifice formed in the gas feed passage between the heating part and the reaction vessel, wherein while the pressure in the heating part is higher than the pressure in the reaction vessel due to pressure loss at the orifice, the process gas is supplied into the heating part via the gas feed passage to preheat the process gas to a predetermined temperature to supply the preheated process gas to the reaction vessel. The heating part may comprise a heating chamber for heating the process gas, and a heater part, provided so as to surround the heating chamber, for heating the heating chamber.
In such a heat treatment system, there is carried out a heat treatment method comprising the steps of: supplying the process gas to a heating part, which is provided outside of the reaction vessel, to preheat the process gas; and feeding the preheated process gas into the reaction vessel, wherein the step of preheating the process gas is carried out while the pressure in the heating part is higher than the pressure in the reaction vessel due to pressure loss at an orifice which is formed in a gas feed passage provided between the heating part and the reaction vessel and which has a smaller inner diameter than that of the gas feed passage.
Thus, even if a low pressure process is carried out in the reaction vessel, the degree of reduced pressure in the heating part is smaller than that in the reaction vessel due to pressure loss at the orifice. Thus, convection sufficiently occurs in the heating part, and the partial pressure of the process gas increases, so that the heating part is sufficiently heated to the inside thereof to improve the heating efficiency of the process gas. Thus, since the process gas can be preheated in the heating part to a predetermined temperature, e.g., a temperature at which the process gas is activated to an extent that the process gas is decomposed, the process temperature can be lowered in the reaction vessel, and the high uniformity of the thickness of a deposited film can be ensured even in such a low temperature process.
According to a further aspect of the present invention, there is provided a heat treatment system wherein an object to be treated is carried in a reaction vessel, the interior of which is heated to a predetermined process temperature, and a process gas is supplied into the reaction vessel via a gas feed passage to process the object, the heat treatment system comprising: a heating part, provided in the gas feed passage, for heating the process gas to a predetermined temperature before the process gas is supplied to the reaction vessel, wherein the gas feed passage arranged between the heating part and the reaction vessel comprise a double pipe comprising an inner pipe and an outer pipe which is provided outside of the inner pipe so as to be spaced from the inner pipe, and wherein the process gas is supplied into the heating part via the gas feed passage to be preheated to a predetermined temperature to be supplied to the reaction vessel via the gas feed passage.
In such a heat treatment system, the gas feed pipe arranged between the heating part and the reaction vessel comprises a double pipe, and the preheated process gas is supplied to the reaction vessel via the inner pipe of the double pipe. Therefore, it is possible to suppress the radiation of the process gas passing through the double pipe, and it is possible to feed the process gas into the reaction vessel while a high temperature is maintained.
The outer pipe of the double pipe of the gas feed passage may be bent to form a flange which is connected to the reaction vessel via a sealing member. In this case, since the temperature in the outer pipe is lower than that in the inner pipe, the gas feed passage can be connected to the reaction vessel without deteriorating the sealing member of, e.g., a resin, due to heat.
According a still further aspect of the present invention, there is provided a heat treatment system wherein an object to be treated is carried in a reaction vessel, which has been pressure-reduced to a predetermined degree of vacuum by evacuating means and the interior of which is heated to a predetermined process temperature, and a process gas is supplied into the reaction vessel via a gas feed passage to process the object, the heat treatment system comprising: a gas chamber which is provided in the gas feed passage and through which the process gas passes; a partition wall for dividing the gas chamber into a plurality of compartments in ventilation directions of the process gases; a vent hole which is formed in the partition wall and which has a smaller inner diameter than that of the gas feed passage; and a heater part, provided so as to a heating chamber, for heating the heating chamber which is an upstream compartment of the plurality of compartments, wherein while the pressure in the heating chamber is higher than the pressure in the reaction vessel due to pressure loss at the vent hole formed in the partition wall, the process gas is supplied into the heating chamber via the gas feed passage to preheat the process gas to a predetermined temperature to supply the preheated process gas to the reaction vessel.
Also in such a construction, the degree of reduced pressure in the heating chamber is smaller than that in the reaction vessel due to pressure loss at the vent hole, so that the heating efficiency of the process gas in the heating chamber is improved.
Preferably, a ventilation resistance material is provided in the heating chamber, the ventilation resistance material contacting the process gas to preheat the process gas to a predetermined temperature. In this case, the heating efficiency of the process gas is further improved.